circle around fighting crosshair?

[Squingus]

whats that for?

[Lieste]

Indicates a 'hard' lock on this target. If you parry someone else and riposte them you will return to the hard-locked opponent.

A soft lock will 'permanently' shift the focus under the same circumstances.

You can cycle all visible 'in range' targets using TAB, or scrolling the mousewheel.

I usually prefer to set the person I want to kill first using the hardlock, but YMMV.
Choosing someone with no shield and lighter armour as your first target results in more reliable quick kills and a simplification of the combat. Saving shielded or heavily armoured opponents to later and working them more cautiously as they are harder to kill quickly. If I can move in such a way to bring an opponent of my chosen protection level to one flank then allows a lateral movement to further simplify the combat by masking his allies behind him, rather than having advancing opponents outpacing you as you back away.